1. Field of the Invention
The present invention generally relates to an electromagnetic induction device such as, for example, a transformer or a choke of a type employing a split-type core assembly.
2. Description of the Prior Art
As a choke and a transformer, there has long been employed a split-type core assembly comprising a plurality of split core segments made of soft magnetizable material. An example of the prior art split-type core assemblies is shown in FIG. 13 of the accompanying drawings for the purpose of discussion of the prior art believed to be pertinent to the present invention.
Referring to FIG. 13, the prior art split-type core assembly 50 for a choke, schematically shown therein in perspective view, comprises a core body split into a pair of core segments 15 of substantially identical shape. Those split core segments 51 have respective windings 52 turned therearound in balanced fashion so that, when electric currents are supplied to the windings 52 on the corresponding split core segments 51 from a suitable source of electric current, the magnetic fields can be developed in respective directions opposite to each other. Although not shown, those split core segment 51 are urged by compression springs in directions opposite to each other shown by the arrow P to contact with each other thereby to complete the split-type core assembly 50.
This prior art split-type core assembly 50 is assembled by preparing the windings 52 wound on respective bobbins, subsequently mounting the bobbins, having the windings 52 wound therearound, on the associated split core segments 51, and finally mating the split core segments 51 together by the application of urging forces to the split core segments 51 in the direction opposite to each other. Therefore, as compared with the choke employing a solid core body to form a closed magnetic path, this type of split-type choke is advantageous in that the split core assembly is inexpensive and can be manufactured easily.
However, this type of split-type core assembly 50 has a problem which will now be discussed. Each of the split core segments 51 so far shown is shaped to represent a generally E-shaped configuration having three arms. When the split core segments 51 are mated together by the application of the urging forces generated by the respective compression springs applied thereto from the directions opposite to each other, split end faces 54 of the arms of those split core segments 51 are brought into contact with each other. As best shown in FIG. 14 showing on an exaggerated scale, one of joints between the respective arms of the split core segments 51, the presence of minute indentations H and/or cracks C at each split end face 54 results in inclusion of air gaps A at the joints between the respective arms of the split core segments 51. Therefore, once the air gaps A are included at the joints between the respective arms of the split core segments 51, the magnetic permeability of the split-type core assembly 50 as a whole tends to be lowered.
By way of example, if the non-split core body exhibits a magnetic permeability of about 10,000, the magnetic permeability will be reduced 50% down to about 5,000 when the core assembly is assembled by the use of the split core segments.
Accordingly, as compared with the non-split core assembly, the split-type core assembly requires the number of turns of each windings 52 which is about 1.4 times that of the winding in the non-split core assembly so that the split-type core assembly can provide an inductance equal to that exhibited by the non-split core assembly. Thus, the increase in number of turns of each windings 52 results in an increase in electric resistance and also in distributed capacitance and, consequently, the characteristic of the choke tends to be lowered.
On the other hand, the magnetic permeability can be increased to 80% of that exhibited by the non-split core assembly if each split end face 54 is surface-finished to a surface smoothness in the order of submicron (specifically not greater than 1.0.mu.m). However, the surface treatment to provide the precisely polished surface on each split end face 54 tends to result in an increase of the manufacturing cost and the resultant core assembly will be expensive.
It can be contemplated that the split core segments 51 are bonded together by the use of a bonding material thereby to complete the split-type core assembly in view of the fact that the bonding material containing ferromagnetic particles of 10.mu.m or greater in particle size has been made available such as disclosed in the Japanese Laid-open Patent publication No. 1-284574. It has, however, been found that, since the particle size of the ferromagnetic particles contained in the bonding material is relatively great, the use of such bonding material in an attempt to connect the split core segments together results in that the air gaps A tend to expand as a result of an inclusion of the relatively large particles in the bonding material, and therefore, unless the bonding material employs particles of high magnetic permeability, the magnetic permeability of the split-core assembly as a whole tends to be lowered.
The foregoing problems discussed above can be equally found in a transformer or choke employing a yoke-mounted core, i.e., employing a core having a yoke (hereinafter referred to as a yoke-mounted core) such as shown in FIG. 15. Referring to FIG. 15, a yoke 63 is secured to the core 62 which is in the form of an open-ended bar (a type wherein no closed magnetic circuit is formed) having primary and secondary windings 60 and 61 formed therearound. The mounting of the primary and secondary windings 60 and 61 on the core 62 can easily be accomplished since the primary and secondary windings 60 and 61 are turned around respective bobbins (not shown) and the core 62 is subsequently inserted through the bobbins prior to the mounting of the yoke 63 to the core 62. However, the presence of minute surface indentations and/or cracks can be found at surface areas 64 and 65 of contact between the yoke 63 and the core 62 and, therefore, by the same reasoning as discussed above, the magnetic permeability of the core assembly as a whole tends to be lowered due to the presence of air gaps.